Some multimedia application data formats use hierarchical layers, with a base layer (BL) and one or more enhancement layers (EL). Data within a layer are often packetized, i.e. organized in packets or frames. While the BL signal alone can be decoded to obtain reproducible multimedia data and comprises all information for a basic decoding, the EL signal comprises additional information that cannot be decoded alone to obtain useful multimedia data. Instead, the EL data are tightly coupled to the BL data, and are only useful together with them. Usually the BL and the EL data are added or superposed to each other, either for a common decoding or after their individual decoding. In either case it is necessary to synchronize the EL data to the BL data, since otherwise the EL data contain no useful information.
It is common to check the integrity of each sub bitstream individually, e.g. by individual CRC check sums or forward error correction (FEC) mechanisms. In general, such CRC or FEC schemes are applied to individual, separate blocks or signal frames of the sub bitstreams. If the decoding starts at synchronized positions in the sub bitstreams, and if in addition for none of the sub bitstreams transmission errors are detected, the decoder will assume that the hierarchical bitstream is in synchronization. However, because this technique does only give certainty regarding the reception of the individually protected blocks of a bitstream, there may still be undetected errors, e.g. if one of the protected blocks of one sub bitstream is completely lost.
Further, it is desirable to keep the data rate as low as possible, which leads to sophisticated data compression methods. A disadvantage of the CRC or FEC approach is that typically a fair amount of additional data has to be transmitted for each block of the bit stream. Therefore, variable length coding is often used for data words that are not equally distributed. In variable length coding (VLC), data words that appear more frequently, i.e. with higher probability, are encoded into shorter code words, while data words that appear with lower probability are encoded into longer code words. Thus, the average amount of bits in encoded messages is shorter than using constant code word length. However, high compression methods such as VLC are more sensitive to bit errors, which may lead to complete data loss. E.g. for VLC it may be impossible to determine which bits belong to a code word when synchronization has been lost. Therefore a common solution to limit possible data loss is the insertion of unique synchronization words that can be recognized with very high probability. However, synchronization words increase the data rate, and the more synchronization words are used, the higher is the data rate. Moreover, a further disadvantage of applying CRC or FEC methods on blocks of sub bitstreams is that it is very difficult to tackle blocks that have varying lengths and no block headers, like in VBR coding.
In order to enable the detection of bit errors in data transmission or storage, it is a common method to calculate a checksum over encoded data, and transmit or store also the checksum. Different types of checksums are known for detecting or for correcting certain bit errors. However, the higher the amount of information in the checksum is, the more bits it requires. Thus, the more useful a checksum is, the more it increases the data rate.